https://github.com/torvalds/linux
Revision 012a45e3f4af68e86d85cce060c6c2fed56498b2 authored by Leon Ma on 30 April 2014, 08:43:10 UTC, committed by Thomas Gleixner on 30 April 2014, 10:34:51 UTC
If a cpu is idle and starts an hrtimer which is not pinned on that same cpu, the nohz code might target the timer to a different cpu. In the case that we switch the cpu base of the timer we already have a sanity check in place, which determines whether the timer is earlier than the current leftmost timer on the target cpu. In that case we enqueue the timer on the current cpu because we cannot reprogram the clock event device on the target. If the timers base is already the target CPU we do not have this sanity check in place so we enqueue the timer as the leftmost timer in the target cpus rb tree, but we cannot reprogram the clock event device on the target cpu. So the timer expires late and subsequently prevents the reprogramming of the target cpu clock event device until the previously programmed event fires or a timer with an earlier expiry time gets enqueued on the target cpu itself. Add the same target check as we have for the switch base case and start the timer on the current cpu if it would become the leftmost timer on the target. [ tglx: Rewrote subject and changelog ] Signed-off-by: Leon Ma <xindong.ma@intel.com> Link: http://lkml.kernel.org/r/1398847391-5994-1-git-send-email-xindong.ma@intel.com Cc: stable@vger.kernel.org Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
1 parent 6c6c0d5
Tip revision: 012a45e3f4af68e86d85cce060c6c2fed56498b2 authored by Leon Ma on 30 April 2014, 08:43:10 UTC
hrtimer: Prevent remote enqueue of leftmost timers
hrtimer: Prevent remote enqueue of leftmost timers
Tip revision: 012a45e
pwm.txt
Pulse Width Modulation (PWM) interface
This provides an overview about the Linux PWM interface
PWMs are commonly used for controlling LEDs, fans or vibrators in
cell phones. PWMs with a fixed purpose have no need implementing
the Linux PWM API (although they could). However, PWMs are often
found as discrete devices on SoCs which have no fixed purpose. It's
up to the board designer to connect them to LEDs or fans. To provide
this kind of flexibility the generic PWM API exists.
Identifying PWMs
----------------
Users of the legacy PWM API use unique IDs to refer to PWM devices.
Instead of referring to a PWM device via its unique ID, board setup code
should instead register a static mapping that can be used to match PWM
consumers to providers, as given in the following example:
static struct pwm_lookup board_pwm_lookup[] = {
PWM_LOOKUP("tegra-pwm", 0, "pwm-backlight", NULL),
};
static void __init board_init(void)
{
...
pwm_add_table(board_pwm_lookup, ARRAY_SIZE(board_pwm_lookup));
...
}
Using PWMs
----------
Legacy users can request a PWM device using pwm_request() and free it
after usage with pwm_free().
New users should use the pwm_get() function and pass to it the consumer
device or a consumer name. pwm_put() is used to free the PWM device. Managed
variants of these functions, devm_pwm_get() and devm_pwm_put(), also exist.
After being requested, a PWM has to be configured using:
int pwm_config(struct pwm_device *pwm, int duty_ns, int period_ns);
To start/stop toggling the PWM output use pwm_enable()/pwm_disable().
Using PWMs with the sysfs interface
-----------------------------------
If CONFIG_SYSFS is enabled in your kernel configuration a simple sysfs
interface is provided to use the PWMs from userspace. It is exposed at
/sys/class/pwm/. Each probed PWM controller/chip will be exported as
pwmchipN, where N is the base of the PWM chip. Inside the directory you
will find:
npwm - The number of PWM channels this chip supports (read-only).
export - Exports a PWM channel for use with sysfs (write-only).
unexport - Unexports a PWM channel from sysfs (write-only).
The PWM channels are numbered using a per-chip index from 0 to npwm-1.
When a PWM channel is exported a pwmX directory will be created in the
pwmchipN directory it is associated with, where X is the number of the
channel that was exported. The following properties will then be available:
period - The total period of the PWM signal (read/write).
Value is in nanoseconds and is the sum of the active and inactive
time of the PWM.
duty_cycle - The active time of the PWM signal (read/write).
Value is in nanoseconds and must be less than the period.
polarity - Changes the polarity of the PWM signal (read/write).
Writes to this property only work if the PWM chip supports changing
the polarity. The polarity can only be changed if the PWM is not
enabled. Value is the string "normal" or "inversed".
enable - Enable/disable the PWM signal (read/write).
0 - disabled
1 - enabled
Implementing a PWM driver
-------------------------
Currently there are two ways to implement pwm drivers. Traditionally
there only has been the barebone API meaning that each driver has
to implement the pwm_*() functions itself. This means that it's impossible
to have multiple PWM drivers in the system. For this reason it's mandatory
for new drivers to use the generic PWM framework.
A new PWM controller/chip can be added using pwmchip_add() and removed
again with pwmchip_remove(). pwmchip_add() takes a filled in struct
pwm_chip as argument which provides a description of the PWM chip, the
number of PWM devices provided by the chip and the chip-specific
implementation of the supported PWM operations to the framework.
Locking
-------
The PWM core list manipulations are protected by a mutex, so pwm_request()
and pwm_free() may not be called from an atomic context. Currently the
PWM core does not enforce any locking to pwm_enable(), pwm_disable() and
pwm_config(), so the calling context is currently driver specific. This
is an issue derived from the former barebone API and should be fixed soon.
Helpers
-------
Currently a PWM can only be configured with period_ns and duty_ns. For several
use cases freq_hz and duty_percent might be better. Instead of calculating
this in your driver please consider adding appropriate helpers to the framework.
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